The growing concern for the protection of the environment has led federal, state and local governments to enact a series of laws and regulations placing strict standards on the permissible percentages of heavy metals in waste waters, solids and solid wastes. Prior to the enactment of these laws, industries that generated solid and liquid wastes containing heavy metals, such as chromium, were not regulated and they disposed of these waste materials with little or no regard for the environmental consequences. Chromium contaminated wastes are generated by several industries, including metal finishing or plating operations, mining operations, milling operations, tanneries and operations using bichromates for processing organic products. Careless handling of such chromium containing materials and wastes has in many cases led to the contamination of the soil in the vicinity of these facilities and elsewhere.
Chromium exists in various oxidation states ranging from valences of 2- to 6+. Hexavalent chrome has a 6+ valence state and forms divalent anions like CrO.sup.4.sup.2- (Chromate) and Cr.sub.2 O.sub.7.sup.2- (dichromate) under acidic conditions. Both chromate and dichromate anions are highly reactive and extremely mobile in soils and waste materials. Chromates and dichromates are strong oxidizing agents that are extremely toxic and hazardous to living organisms. For example, chromic acid and sodium chromates are used in laboratory for the oxidation of organics compounds. Therefore, hexavalent chromium that is discovered in soils or exists as an industrial waste material must be destroyed, contained and/or stabilized to reduce the risks to the environment caused by its rapid migration and highly toxic characteristics.
The Resource Conservation and Recovery Act of 1976, commonly known as the RCRA, provided for federal classification of hazardous waste. The statutory language defines "hazardous waste" as solid waste or combinations of solid waste which pose a "substantial present or potential hazard . . . when improperly treated, stored, transported, or disposed of, or otherwise mismanaged." Any solid waste that exhibits one of the hazard characteristics defined in subpart C of Part 261, Volume 40, Code of Federal Regulations is, by definition, a hazardous waste.
A solid waste is considered to be a hazardous waste if it is listed by the Environmental Protection Agency (EPA), or it exhibits characteristics of either ignitability, corrosivity, reactivity, or toxicity as determined by the Toxicity Characteristic Leaching Procedure (TCLP) (USEPA Method 1311). Historically, toxicity characteristic regulations had been based on the Extraction Procedure (EP) Toxicity Test (USEPA Method 1310), which specified laboratory steps to be followed in analyzing samples. The test was aimed at identifying the tendency of wastes to generate a leachate with concentrations of contaminants greater than the values listed in Appendix II of the code of Federal Regulations, Part 261.24, page 406, revised Jul. 1, 1988. If concentrations of leachable, mobile chromium were found to be greater than 5 milligrams per liter, the material was considered characteristically toxic for chromium and hence hazardous with respect to chromium content. Such characteristically toxic wastes required treatment to comply with the USEPA regulations defining the treatment standards for chromium and other parameters of concern. This EP Toxicity Test is now obsolete, and has been replaced by the TCLP test for 39 different parameters including chromium.
Effective Nov. 8, 1990, the USEPA established the treatment standard for chromium wastes (DOO7), and particularly for chromium contaminated soils and solid wastes, at a toxicity characteristic level of 5 milligrams per liter in the extraction fluid according to the TCLP test. The TCLP test is much more rigorous--and is more uniformly applicable to a larger number of parameters--than the EP Toxicity Test. It replaced the EP toxicity method for RCRA waste determination. The TCLP test requires sizing of waste material to less than 3/8 inches or 9.5 mm and agitation of a 100 g waste sample in 2 liters of specified extraction fluid for 18 hours on a rotating agitator at a speed of about 30 revolutions per minute. The chromium concentration is determined in the extraction fluid after filtration under pressure, and expressed in units of milligrams per liter (mg/l).
Chromium occurs in aqueous systems as both the trivalent (Cr.sup.+3) and the hexavalent (Cr.sup.+6) ions. Chromium is present in industrial wastes primarily in the hexavalent form, as chromate (CrO.sub.4.sup.-2) and dichromate (Cr.sub.2 O.sub.7.sup.-2). Chromium compounds are added to cooling water to inhibit corrosion. They are employed in the manufacture of inks and industrial dyes and paint pigments, as well as in chrome tanning, aluminum anodizing, and other metal cleaning, preplating, and electroplating operations. Chromates are also contained in some preservatives and fire-retardant chemicals used in wood preservative treatments. Automobile parts manufacturers are one of the largest producers of chromium-plated metal parts. Frequently the major source of waste chromium is the chromic acid bath and rinsewater used in such metal-plating operations. Reduction of hexavalent chromium from a valence state of plus six to plus three, and subsequent hydroxide precipitation of the trivalent chromic ion, is the most common method of hexavalent chromium control. To meet increasingly stringent effluent standards, some industries have turned to ion exchange to treat chromate and chromic acid wastes. Evaporative recovery of concentrated chromate and chromic acid wastes has also proved technically and economically feasible as a pollution abatement alternative. The application of other processes, such as electrochemical and activated-carbon adsorption techniques, is receiving increasing attention.
U.S. Pat. No. 5,009,793, by Muller relates to the process of metal separation by adjustment of the pH in the range of 3.5 to 11 so that the dissolved metal salts are precipitated as metal hydroxides. The pH adjustment is initially made with an acid and then adjusted with the milk of lime in the presence or absence of hydrogen peroxide as an aid to oxidation and hydroxylation. Under natural conditions, this process is reversible and the precipitated chromium can convert back to hexavalent chrome. In U.S. Pat. No. 5,000,858, Manning and Wells discuss a process for removing hexavalent chromium from water. The process comprises the steps of: decreasing the pH value to below 3, adding a reducing agent to convert hexavalent chromium to chromic ion, increasing the pH in order to precipitate the metal hydroxide, adding an anionic polymer as a flocculant to settle the trivalent precipitate, and removing the solids to achieve a treatment goal of 0.05 mg/l chromium in the treated water. This process is cumbersome and generates a hazardous solid waste by the TCLP test criteria.
U.S. Pat. No. 4,684,472 by Abbe and Cole relates to the reduction of soluble chromium with sulfide salts to generate a solid precipitate and an aqueous fraction. This method generates a chromium sulfide sludge which under natural environmental conditions due to biological activity can oxidize into chromates and sulfates. In U.S. Pat. No. 3,981,965, Gancy and Wamser discuss a method of treating solid waste material with sulfide ion to convert soluble chromium to an insoluble state. The treatment additives include calcium sulfide, sodium hydrosulfite, dithionates, dithionites, thiourea, thioglycolic acid and sodium xanthates. These reducing compounds convert hexavalent chrome (Cr.sup.6+) into trivalent chromium (Cr.sup.3+) and divalent chromium (Cr.sup.2+) forms which under oxidative conditions may revert back to hexavalent chromium.
U.S. Pat. No. 4,678,584, by Elfine relates to the use of trithiocarbonates, especially Na.sub.2 CS.sub.3 and CaCS.sub.3 to precipitate metals as insoluble sulfides. A mole of trithiocarbonate removes a mole of heavy metal from waste water and forms a metal bearing solid waste and an effluent relatively free of metals. However, under natural environmental conditions these metal sulfides may oxidize, transforming chromates of various metal species into soluble and mobile chromium.
Solidification methods based on Portland cement (see U.S. Pat. No. 4,741,776 to Bye), pozzolans, lime kiln dust, calcium carbonate and powdered lime for chromium fixation are temporary solutions. Furthermore, these methods increase waste volume and mass, and therefore, only dilute the chromium in the final waste matrix. In U.S. Pat. No. 3,201,268, Hemwall discusses the stabilization of soils having lead salts by the addition of phosphoric acid. The use of phosphoric acid alone for chromium fixation of waste media and process materials containing hexavalent chromium does not work because the treated waste fails to pass the TCLP test criteria.
The prior art methods are temporary solutions, generally applicable to waste waters. The need exists for a cost-effective permanent treatment technology which can be applied to solid wastes, precipitated solids, soils, sand, gravel, sludges, waste waters and other waste media containing hexavalent chrome.